The present invention relates to a charged particle beam apparatus and, more particularly, to a charged particle beam apparatus in which a primary charged particle beam is allowed to easily pass through the center either of an objective aperture or of an aperture present within the orbit in which the charged particle beam passes, whereby the apparatus is adapted to obtain a high-brightness image stably.
In a charged particle beam apparatus typified by a scanning electron microscope, a sharply focused primary charged particle beam is scanned over a sample to obtain information (e.g., an image of the sample) from the sample. The microscope column of such a charged particle beam apparatus has an objective aperture for appropriately limiting the current hitting the sample (probe current). If there is a deviation between the center of the objective aperture and the center of the primary charged particle beam, the amount of the probe current of the primary charged particle beam limited by the objective aperture increases, thus decreasing the brightness of the sample image. In this case, appropriate control of the probe current cannot be achieved. It follows that a sample image of desired brightness cannot be obtained. Therefore, accurate adjustment is necessary to obtain a sample image of normal brightness by causing the primary charged particle beam to pass through the center of the objective aperture at all times. This adjustment is hereinafter referred to as the beam center axis adjustment.
Usually, in the beam center axis adjustment, a dedicated deflector is disposed over the objective lens. The primary charged particle beam is scanned over the objective aperture by the deflector, and an image of the objective aperture is obtained. An example of image of the objective aperture hole is shown in FIG. 2A. This image is created by a secondary signal produced by bombardment of the sample with the primary charged particle beam passed through the objective aperture. The center of the whole image (scanning center) is the center of the beam of the primary charged particle beam. The image of the white circular image is the image of the objective aperture hole. The center of the circle is the center of the objective aperture hole. In FIG. 2A, the center of the white circle deviates from the center of the image. Therefore, it can be seen that under this condition, the center of the primary charged particle beam does not pass through the center of the objective aperture hole during observation of the sample image.
Under the condition shown in FIG. 2A, to cause the primary charged particle beam to pass through the center of the objective aperture hole, the primary charged particle beam is moved into the center of the objective aperture hole, normally using a deflector (aligner) for adjustment of the beam center axis, the aligner being placed over the objective aperture. At this time, the image of the objective aperture is as shown in FIG. 2B. It is possible to bring the center of the whole image (scanning center) into coincidence with the center of the image of the objective aperture hole.
In the past, the aligner for adjustment of the beam center axis has been set manually by an operator while watching the image (image for adjustment) of the objective aperture hole. In one available method, means for storing aligner-setting conditions in relation to the optical conditions of the charged particle beam apparatus is mounted. During manipulation, the aligner-setting conditions are read out, and setting is done. Furthermore, to automate the alignment, a technique for grasping the amount of axial deviation by image processing is disclosed in JP-A-2005-310699.
Furthermore, in a charged particle beam apparatus typified by a scanning electron microscope, a sharply focused charged particle beam is scanned over a sample, and desired information (e.g., an image of the sample) is obtained from the sample. In such a charged particle beam apparatus, if the optical axis deviates from the lens, lens aberration is produced, deteriorating the resolution of the sample image. Consequently, accurate axial adjustment is required in order to obtain a sample image at a high resolution. Therefore, in the prior-art axial adjustment, the excitation current of the objective lens or the like is varied periodically. The conditions under which the deflector (aligner) for axial adjustment operates are manually adjusted to minimize the motion produced at that time. A technique for automating such an adjustment is disclosed in JP-A-2002-352758 corresponding to U.S. Pat. No. 6,864,493.
This description reports a technique of a method of automatic axial adjustment. That is, under some deflection conditions of the alignment deflector, the objective lens conditions are varied to two conditions. The resulting image deviation is detected. The deviation between the images at two locations is applied to an equation. Optimum alignment conditions are found, and settings are made.
Furthermore, if there is a deviation from the center of the stigmator (astigmatic corrector) that performs astigmatic correction of the charged particle beam, the field of view is moved when astigmatism is adjusted. This makes it difficult to make the adjustment. Therefore, another aligner (deflector) for controlling the position of charged particles on the sample in an interlocking manner to the operation of the astigmatic corrector is provided. Motion of the image in response to variation of the set value of the stigmator (astigmatic corrector) is canceled out. Thus, the field of view is corrected such that the observed image does not move during adjustment of the astigmatism. A technique for automating this adjustment is also disclosed in JP-A-2002-352758 corresponding to U.S. Pat. No. 6,864,493.
There is a report of a technique for a method of automatic axial adjustment. In particular, the objective lens conditions are varied to two sets of conditions in some stigmator-deflecting conditions in the same way as the foregoing. The resulting deviation between images is detected. The deviation between the images at two locations is applied to an equation. Optimum alignment conditions are found, and settings are made.